Plasma arc torches generally include a metallic electrode and a nozzle assembly positioned adjacent the discharge end of the electrode. These torches typically operate in a transferred arc mode where the arc extends from the discharge end of the electrode through the nozzle to a workpiece. An oxidizing gas normally is used in the torch for plasma generation and for facilitating faster and more efficient cutting of the workpiece.
However, because of the high voltages required for starting and transferring the arc from the electrode to the workpiece, some prior art torches typically are started with a non-oxidizing gas by creating a pilot arc between the discharge end of the electrode and the nozzle assembly. If an oxidizing gas were used during the starting process, the high voltages used would create severe oxidation conditions and reduce the effective electrode life. This generated arc then is transferred to the workpiece When the arc is transferred, the flow of non-oxidizing gas is reduced and an oxidizing gas such as oxygen is added to the flow of the non-oxidizing gas.
Generally, the aforementioned prior art method requires careful control and timing of the gas flow and in some torches requires a special torch structure. For example, in one torch design, argon flows through multiple annular gas ports positioned between two nozzle members during initial arc starting. When the arc has transferred, some argon flow in the gas ports is terminated and substituted with a flow of oxidizing gas so that during the transferred torch operation, a reduced flow of argon is mixed with an oxidizing gas. This use of a combination of argon and oxygen or air within the torch requires simultaneous, complex control over two different gas flows for maintaining proper mixing and operation of the torch. Additionally, a non-oxidizing gas such as argon results in the increased formation of dross when the non-oxidizing gas is mixed with oxygen or air in a transferred plasma arc torch operation during a cutting operation.
It is therefore an object of the present invention to provide a process for starting a plasma arc torch of the described type which minimizes the oxidization of an electrode by providing a flow of non-oxidizing gas during pilot arc creation and a new, substituted flow of oxidizing gas only during transferred plasma arc torch operation.